Known methods for producing a biaxially oriented resin film such as a biaxially oriented polyester film include a sequential biaxial stretching method comprising the steps of continuously discharging a flowable resin from a die as a sheet, cooling and solidifying the discharged sheet on a casting drum, to form a cast film, stretching the formed cast film in the carrying direction of the film, namely, in the machine direction using a longitudinal stretching machine and subsequently stretching the film stretched in the machine direction (monoaxially oriented film) in the width direction of the film (transverse direction) in a tenter oven, and a simultaneous biaxial stretching method of stretching the cast film in the carrying direction of the film (machine direction) and in the width direction of the film (transverse direction) in a tenter oven.
In the tenter oven used for these production methods, installed are air ejection nozzles having many air ejection holes formed in the surfaces thereof to face a surface of a resin film passing through the tenter oven. Usually, multiple air ejection nozzles are installed at regular intervals in the resin film carrying direction in such a manner that the longitudinal direction of the air ejection nozzles is kept perpendicular to the resin film carrying direction.
The air ejection nozzles are provided in a nozzle housing. The nozzle housing has an air supply passage therein and has an air ejection face as one of the surfaces thereof. The ends on one side of the many air ejection holes are opened in the air ejection face, and the ends on the other side are opened into the air supply passage. To another surface or other two surfaces of the nozzle housing, an air supply duct is connected, and one end of the air supply duct communicates with the air supply passage in the nozzle housing while the other end is connected with a heat exchanger and a fan. The air controlled to a desired temperature by the heat exchanger is sent by the fan to the respective air ejection holes through the air supply duct and the air supply passage in the nozzle housing, and is ejected toward the surface of the resin film from the respective air ejection holes open in the air ejection face of the housing. The ejected air is usually collected from suction ports formed in the tenter oven, to be reused.
In general, the tenter oven has multiple divisional zones such as a preheating zone, stretching zone, heat setting zone and cooling zone in the resin film carrying direction. The tenter oven has such a structure that the temperatures of the air used in the respective zones can be set independently for the respective zones. The tenter oven is provided with numerous clips outside both the edges of the resin film for holding the edges of the resin film moving along rails from the inlet portion toward the outlet portion of the tenter oven.
In the tenter oven, the resin film held at both the edges thereof and carried by the clips is heated in the preheating zone to a temperature suitable for stretching, and stretched at least in the transverse direction in the stretching zone, then being heat-treated in the heat setting zone, cooling zone, etc. The air ejection nozzles are used to eject the air controlled at a desired temperature toward the surface of the resin film, for promoting the heat exchange between the air and the resin film, to thereby heat, cool or heat-retain the resin film.
The properties of the resin film produced like this are affected by the heat history which the resin film encounters while it passes through the respective zones of the tenter oven. Therefore, to obtain a resin film having uniform properties in the width direction of the resin film, it is important that the heat exchange between the air ejected from the air ejection nozzles and the resin film takes place uniformly in the width direction of the resin film. For this purpose, the air ejection nozzles are required to assure that the temperature of the air striking the resin film is uniform in the width direction of the resin film and that the heat transfer efficiency of the air ejection nozzles is uniform in the width direction of the resin film.
An air ejection nozzle having a continuous air ejection hole formed in the width direction of the resin film on the surface thereof facing the surface of the resin film is called a slit nozzle. As a conventional slit nozzle for the purpose of keeping the ejection velocity and temperature of air uniform in the width direction of the resin film, a nozzle having a duct of countercurrent flow design is proposed (see Patent Literature 1). However, a slit nozzle has a problem that the air jet is liable to bend in the progress direction. If the air jet bends in the progress direction, air masses different in temperature are mixed at a portion where zones different in set temperature contact each other, and large temperature irregularity can occur in the width direction of the resin film. In this case, it is difficult to obtain a resin film having uniform properties in the width direction.
According to the finding by the present inventors, the abovementioned problem that the air jet is liable to bend in the progress direction can be improved by arranging air ejection holes discretely in the width direction of the resin film, that is, by arranging many air ejection holes independent of each other at regular intervals. The reason is considered to be that the air jets finely divided in the width direction of the resin film form air passing portions between the respectively adjacent air jets, such that the air existing in the front and back of the air ejection nozzle can be guided to pass through the air passing portions, to ease the difference of the pressures in the front and back of the air ejection nozzle. As such an air ejection nozzle, there is a hole nozzle having many circular air ejection holes in the face thereof facing the surface of the resin film. However, if many air ejection holes are arranged at regular intervals in the width direction of the resin film, the heat transfer rate of the surface of the resin film becomes uneven in the width direction of the resin film, to raise a problem that the uniformity of heat transfer efficiency declines.
Proposed is a conventional hole nozzle for the purpose of enhancing the heat transfer rate of the surface of the resin film, in which while the distance between the air ejection holes and the surface of the resin film is set at 4 to 6 times the diameter of the air injection holes, many such air ejection holes are arranged zigzag in six rows in the direction perpendicular to the resin film carrying direction (see Patent Literature 2). However, the magnitude of heat transfer rate and the uniformity of heat transfer efficiency in the width direction of the resin film are different problems, and it is difficult to improve the uniformity of heat transfer efficiency in the width direction of the resin film only by discussing the diameter of air ejection holes and the number of rows of air ejection holes.
The conventional hole nozzles intended to improve the uniformity of heat transfer efficiency in the width direction of the resin film include a hole nozzle used as a device for cooling the resin film on a casting drum (see Patent Literature 3) and a hole nozzle used as a drying device of a printing machine or coating machine (see Patent Literature 4). However, these hole nozzles are effective in the case where the distance between the air ejection holes and the surface of the resin film is made shorter than 20 mm, and it is not preferred to use such hole nozzles in a tenter oven in which the distance between the air ejection holes and the surface of the resin film is generally 140 to 270 mm, since the heat transfer rate of the surface of the resin film may decline remarkably.
Patent Literature 1: JP 1634915 B
Patent Literature 2: JU 2528669 B
Patent Literature 3 JP 3374527 B
Patent Literature 4: JU 2008679 B